Some questions with tenses (Simple Past/Past Perfect; Present Perfect/Present Perf. Cont.) Why is simple past okay here? I thought past perfect was mandatory, because the second part of the sentence happened before the first? -> "She showed him the letter she had received / received from her brother." Same here: "I knew the poem well because we were studying / had been studying it in our literature class." And here: "I wanted to meet Mr De Jong because he had known / knew my grandfather during the war." (Are they differing in meaning at all?) And why are both tense forms ok here, too? "I have looked / have been looking forward to this concert for ages." Thank you :)